1. Field of the Invention
This invention relates to an apparatus for measuring a diffused layer depth of a semiconductor substrate, and more particularly to an apparatus suitable for measuring a diffusion depth of a diffused wafer used for manufacturing a discrete semiconductor such as a high output transistor or diode, etc.
2. Description of the Prior Art
In manufacturing of so called power devices such as high output transistors, diodes, or rectifying elements, etc., there is used a diffused wafer in which impurities are diffused at a high concentration at the back surface of a silicon wafer. The high concentration diffused layer of such a diffused wafer serves to reduce the series resistance at the time of forming a collector electrode of a transistor formed by using this wafer, or to reduce the ohmic contact resistance of the electrode metal.
Such a diffused wafer is conventionally manufactured as follows. First, an n.sup.- type silicon substrate including phosphorus (P) doped at a low concentration, and having a resistivity of 100 .OMEGA.cm and a thickness of 1000 .mu.m is prepared (FIG. 4A). Then, phosphorus (P) is diffused at a high concentration into such n-type silicon substrate from the surface thereof by using POCl.sub.3 as a doping gas in the atmosphere of a temperature of about 1200.degree. C. so that the diffusion depth x.sub.j becomes about 100 .mu.m, thus to form n.sup.+ layers 21 at the both surfaces of the silicon substrate (FIG. 4B).
Since the low concentration diffused layer 20 serves as the area where regions performing device function such as emitter, base or collector, etc. of a transistor are to be formed, one side of the silicon substrate is ground so that the thickness I.sub.so of the low concentration diffused layer 20 is equal to 20 to 150 .mu.m to agree with design requirements of the element, e.g., withstand voltage or current amplification factor, etc. As a result, the total thickness of the thickness x.sub.j of the high concentration diffused layer 21 and the thickness I.sub.so of the low concentration diffused layer 20 is equal to a thickness of several hundreds .mu.m having a strength to an extent such that no crack occurs in handling of the wafer.
By making use of the diffused wafer obtained in this way to repeatedly implement, e.g., surface oxidation process, PEP process for forming a window for diffusion, impurity diffusion process are conducted with respect to the base and the collector to form necessary electrodes, wirings(interconnections), or a protective film or films, etc. Thus, a semiconductor device including a bipolar transistor is fabricated.
FIGS. 5A and 5B are graphs showing an impurity concentration profile in a thickness direction of the wafer. In FIG. 5A corresponding to the state of FIG. 4B, depth is taken in an x-axis direction. When such a curve is determined, it is possible to precisely obtain thicknesses of x.sub.j and I.sub.so.
As one method for determining such an impurity concentration profile, there is known a method in which a p-type wafer of an opposite conductivity type is used to carry out diffusion, then the wafer surface is polished by the cylinder lapping method or the ball lapping method to take a cylindrical or spherical form so that the p-n diffusion surface is exposed to apply staining to allow the p-n junction to be visualized to measure the thickness of the n.sup.+ layer by using vernier scale.
FIG. 6 shows the state where the wafer surface is polished so as to take a form of sphere having a radius R. From the visualized position of the p-n junction, a and b are determined or calculated. By making use of these values, the thickness x.sub.j can be determined or calculated by the following formula: EQU x.sub.j =R{(1-b.sup.2 /R.sup.2).sup.1/2 -(1-a.sup.2 /R.sup.2).sup.1/2 }
Furthermore, in order to examine the accuracy of an actual finish of the diffused wafer, the following method is used. A test piece 40 cut from a sample wafer sampled from a modulus lot is secured by means of wax on a jig having a base 31 inclined at an angle as shown in FIG. 7. Then the sample piece is obliquely polished by using a solution including abrasive or polishing agent soluted therein on a flat glass plate. The distribution of resistance values is determined while scanning the inclined surface by using two probes 43 from the n.sup.- region 41 to the n.sup.+ region 42 as shown in FIG. 8. Then, an intersecting point of the line L1 indicating the n.sup.- region and the extrapolation tangential L2 is determined as shown in FIG. 9. From this position, the Iso thickness of the n.sup.- is determined.
A desired non-diffused layer thickness I.sub.so of the diffused wafer is precisely obtained by determining, as a polishing or abrasive clearance, the thickness obtained by subtracting total thickness of the diffusion depth x.sub.j and the non-diffused layer thickness I.sub.so obtained by the above-described method from the thickness of a wafer which does not undergo diffusion processing to implement polishing or abrasion thereto.
However, it is difficult to determine a precise diffused depth by using the above-described measurement method.
For example, in the polishing of the inclined surface, extremely high skillfulness is required in the work for securing a sample wafer onto a jig by wax or uniform polishing, etc. Furthermore, also in the measurement by two probes, if the n.sup.- region has a resistance of more than 50 .OMEGA.cm caused a small scratch on the polished surface or absorption of abrasive, the resistivity curve in the n.sup.- region does not become flat. For this reason, it is difficult to precisely determine the boundary point between the n.sup.- region and the n.sup.+ region, giving rise to an error in determination of the n.sup.- region thickness.
Furthermore, in the measurement by two probes, as shown in FIG. 10, a measured value is converted to an impurity concentration to use it as a value on the ordinate to draw a tangential at a specific concentration point, thus making it possible to determine the thickness of the n.sup.- region. In this case, however, the criterion of a specific concentration (two terminal resistivity) varies depending upon the wafer maker or user. Therefore, correct measurement is not carried out. Particularly, in the case where the I.sub.so thickness is about 10 m, an error of as large as 50% occurs depending upon the way the tangential is drawn.
In addition, since it is required in the conventional method to prepare a special sample for measurement, an extremely troublesome work is required, resulting in the problem that it takes a long time, e.g., about 2 hours to complete the measurement.